Gasification is a process that enables the conversion of carbonaceous feedstock, such as municipal solid waste (MSW) or coal, into a combustible gas. The gas can be used to generate electricity, steam or as a basic raw material to produce chemicals and liquid fuels.
Possible uses for the gas include: the combustion in a boiler for the production of steam for internal processing and/or other external purposes, or for the generation of electricity through a steam turbine; the combustion directly in a gas turbine or a gas engine for the production of electricity; fuel cells; the production of methanol and other liquid fuels; as a further feedstock for the production of chemicals such as plastics and fertilizers; the extraction of both hydrogen and carbon monoxide as discrete industrial fuel gases; and other industrial applications.
Generally, the gasification process consists of feeding carbonaceous feedstock into a heated chamber (the gasifier) along with a controlled and/or limited amount of oxygen and optionally steam. In contrast to incineration or combustion, which operate with excess oxygen to produce CO2, H2O, SOx, and NOx, gasification processes produce a raw gas composition comprising CO, H2, H2S, and NH3. After clean-up, the primary gasification products of interest are H2 and CO.
Useful feedstock can include any municipal waste, waste produced by industrial activity and biomedical waste, sewage, sludge, coal, heavy oils, petroleum coke, heavy refinery residuals, refinery wastes, hydrocarbon contaminated soils, biomass, and agricultural wastes, tires, and other hazardous waste. Depending on the origin of the feedstock, the volatiles may include H2O, H2, N2, O2, CO2, CO, CH4, H2S, NH3, C2H6, unsaturated hydrocarbons such as acetylenes, olefins, aromatics, tars, hydrocarbon liquids (oils) and char (carbon black and ash).
As the feedstock is heated, water is the first constituent to evolve. As the temperature of the dry feedstock increases, pyrolysis takes place. During pyrolysis the feedstock is thermally decomposed to release tars, phenols, and light volatile hydrocarbon gases while the feedstock is converted to char.
Char comprises the residual solids consisting of organic and inorganic materials. After pyrolysis, the char has a higher concentration of carbon than the dry feedstock and may serve as a source of activated carbon. In gasifiers operating at a high temperature (>1,200° C.) or in systems with a high temperature zone, inorganic mineral matter is fused or vitrified to form a molten glass-like substance called slag. Since the slag is in a fused, vitrified state, it is usually found to be non-hazardous and may be disposed of in a landfill as a non-hazardous material, or sold as an ore, road-bed, or other construction material. It is becoming less desirable to dispose of waste material by incineration because of the extreme waste of fuel in the heating process and the further waste of disposing, as a residual waste, material that can be converted into a useful syngas and solid material.
The means of accomplishing a gasification process vary in many ways, but rely on four key engineering factors: the atmosphere (level of oxygen or air or steam content) in the gasifier; the design of the gasifier; the internal and external heating means; and the operating temperature for the process. Factors that affect the quality of the product gas include: feedstock composition, preparation and particle size; gasifier heating rate; residence time; the plant configuration including whether it employs a dry or slurry feed system, the feedstock-reactant flow geometry, the design of the dry ash or slag mineral removal system; whether it uses a direct or indirect heat generation and transfer method; and the syngas cleanup system. Gasification is usually carried out at a temperature in the range of about 650° C. to 1200° C., either under vacuum, at atmospheric pressure or at pressures up to about 100 atmospheres.
There are a number of systems that have been proposed for capturing heat produced by the gasification process and utilizing such heat to generate electricity, generally known as combined cycle systems.
The energy in the product gas coupled with substantial amounts of recoverable sensible heat produced by the process and throughout the gasification system can generally produce sufficient electricity to drive the process, thereby alleviating the expense of local electricity consumption. The amount of electrical power that is required to gasify a ton of a carbonaceous feedstock depends directly upon the chemical composition of the feedstock.
If the gas generated in the gasification process comprises a wide variety of volatiles, such as the kind of gas that tends to be generated in a low temperature gasifier with a “low quality” carbonaceous feedstock, it is generally referred to as off-gas. If the characteristics of the feedstock and the conditions in the gasifier generate a gas in which CO and H2 are the predominant chemical species, the gas is referred to as syngas. Some gasification facilities employ technologies to convert the raw off-gas or the raw syngas to a more refined gas composition prior to cooling and cleaning through a gas quality conditioning system.
Utilizing plasma heating technology to gasify a material is a technology that has been used commercially for many years. Plasma is a high temperature luminous gas that is at least partially ionized, and is made up of gas atoms, gas ions, and electrons. Plasma can be produced with any gas in this manner. This gives excellent control over chemical reactions in the plasma as the gas might be neutral (for example, argon, helium, neon), reductive (for example, hydrogen, methane, ammonia, carbon monoxide), or oxidative (for example, oxygen, carbon dioxide). In the bulk phase, a plasma is electrically neutral.
Some gasification systems employ plasma heat to drive the gasification process at a high temperature and/or to refine the offgas/syngas by converting, reconstituting, or reforming longer chain volatiles and tars into smaller molecules with or without the addition of other inputs or reactants when gaseous molecules come into contact with the plasma heat, they will disassociate into their constituent atoms. Many of these atoms will react with other input molecules to form new molecules, while others may recombine with themselves. As the temperature of the molecules in contact with the plasma heat decreases all atoms fully recombine. As input gases can be controlled stoichiometrically, output gases can be controlled to, for example, produce substantial levels of carbon monoxide and insubstantial levels of carbon dioxide.
The very high temperatures (3000 to 7000° C.) achievable with plasma heating enable a high temperature gasification process where virtually any input feedstock including waste in as-received condition, including liquids, gases, and solids in any form or combination can be accommodated. The plasma technology can be positioned within a primary gasification chamber to make all the reactions happen simultaneously (high temperature gasification), can be positioned within the system to make them happen sequentially (low temperature gasification with high temperature refinement), or some combination thereof.
The gas produced during the gasification of carbonaceous feedstock is usually very hot but may contain small amounts of unwanted compounds and requires further treatment to convert it into a useable product. Once a carbonaceous material is converted to a gaseous state, undesirable substances such as metals, sulfur compounds and ash may be removed from the gas. For example, dry filtration systems and wet scrubbers are often used to remove particulate matter and acid gases from the gas produced during gasification. A number of gasification systems have been developed which include systems to treat the gas produced during the gasification process.
These factors have been taken into account in the design of various different systems which are described, for example, in U.S. Pat. Nos. 6,686,556, 6,630,113, 6,380,507; 6,215,678, 5,666,891, 5,798,497, 5,756,957, and U.S. Patent Application Nos. 2004/0251241, 2002/0144981. There are also a number of patents relating to different technologies for the gasification of coal for the production of synthesis gases for use in various applications, including U.S. Pat. Nos. 4,141,694; 4,181,504; 4,208,191; 4,410,336; 4,472,172; 4,606,799; 5,331,906; 5,486,269, and 6,200,430.
Prior systems and processes have not adequately addressed the problems that must be dealt with on a continuously changing basis. Some of these types of gasification systems describe means for adjusting the process of generating a useful gas from the gasification reaction. Accordingly, it would be a significant advancement in the art to provide a system that can efficiently gasify carbonaceous feedstock in a manner that maximizes the overall efficiency of the process, and/or the steps comprising the overall process.
As noted above, gas from a gasification system can be exploited for a variety of applications such as the conversion of the gas to energy in the form of electricity or chemical applications such as fuel cells or chemical feedstock. The equipment, which is used to directly convert gas into electricity currently comprises gas turbines and gas engines. These machines are designed to function within a very strict range of characteristics and are often very sensitive to changes in certain gas characteristics. In addition to affecting the efficiency of engine operation, a deviation in the gas characteristics may even have a negative effect on engine operation. For example, changes in the gas characteristics can affect the emissions, efficiency, knock and combustion stability, as well as increase the maintenance requirements of the engine. Accordingly, these gas-utilizing machines work most effectively when the characteristics of the gas are maintained within the specified limits.
The characteristics of the gas produced by a gasification system, such as chemical composition, flow rate, temperature, pressure, and relative humidity will naturally vary over time largely due to variations in the feedstock composition and the reaction conditions that occur, for example, during the gasification process. Some characteristics of the gas will vary on a minute-to-minute basis and some characteristics on a second-to-second basis. A steady stream of gas with consistent characteristics will be produced only if the gas is allowed to mix thoroughly to ensure a homogeneous gas composition and the other characteristics such as temperature, pressure and flow rate are adjusted.
U.S. Pat. No. 6,398,921 describes a gasification process for producing fuel gas for use in internal combustion engines for the generation of electricity. Prior to fueling the engine, the fuel gas is cleaned, compressed, and stored in a tank for limited surge storage. Although the fuel gas is regulated to the inlet pressure required for the engine, the fuel gas is not regulated for other characteristics, namely its composition. Accordingly, there remains a need for a gas homogenization system, which minimizes variance in the gas characteristics (composition, flow rate, pressure, temperature), thereby rendering a steady stream of gas of consistent quality required by downstream machinery.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the invention.